The present invention relates generally to liquid crystal display devices; and, more particularly, the invention relates to a liquid crystal display device that maintains a uniform gap for a liquid crystal layer, to a liquid crystal display device that can prevent optical leakage in the display area, and to a liquid crystal display device that can prevent pollution in the liquid crystal composite material by the seal agent.
Recently, display devices using a liquid crystal panel have become more widely employed as display devices which are capable of visually producing high-precision color images adaptable for use in display devices of the projection type, in notebook personal computers, in monitor units and in other similar visual representation instruments.
Currently available display devices using such a liquid crystal panel (liquid crystal display devices) typically include those of the simple matrix type, which make use of a liquid crystal panel having a liquid crystal layer sandwiched between a pair of substrates with parallel electrodes formed on respective inner surfaces in a mutual crossover fashion, and other panels of the active matrix type which employ a liquid crystal element (referred to also as "liquid crystal panel" hereinafter) that has switching elements for selection in units of pixels on only one of the pair of substrates.
Active-matrix liquid crystal display devices are generally categorized into two groups: one group includes certain liquid crystal display devices of the so-called vertical electric-field type typically including the twisted nematic (TN) scheme (also known as TN active-matrix liquid crystal display devices) configured to include an ensemble of pixel selection electrodes formed on each of a pair of upper and lower substrates, and a second group includes the so-called "lateral electric-field" liquid crystal display devices (generally called in-plane field type IPS liquid crystal display devices) using a specific liquid crystal panel with pixel select electrodes formed on only one of a pair of upper and lower substrates.
Projection liquid crystal display devices are also known as one type of liquid crystal display device application equipment. Projection liquid crystal display devices include an optical system for magnification of an image generated on a liquid crystal panel of small size to provide an enlarged image which is then projected onto a spaced-apart second screen of large size. Such projection liquid crystal display devices include devices of the transmission type and those of the reflection type, the former being designed such that two separate dielectric substrates making up a liquid crystal panel are both formed of transparent substrates, such as glass substrates by way of example, for permitting rays of light to be emitted from the back surface thereof to thereby cause resulting modulated transmission light images to be projected with enlarged sizes on an associative screen by use of an optical lens or combination thereof. On the other hand, the reflective projectors employ one of such dielectric substrates as a reflector plate for emitting light from the surface side to thereby produce an image which consists of modulated reflected light, which in turn is projected by an optical system on a screen with a magnified scale.
There are also display devices for use with notebook PCs or direct view liquid crystal display devices for display monitors, which are designed to employ as a reflector plate either one of the dielectric substrates making up the liquid crystal panel and which utilizes incoming light from the display surface side.
Typically, a liquid crystal panel constituting such a liquid crystal display device is arranged so that a liquid crystal layer made of a chosen liquid crystal material is sandwiched in a gap between two separate dielectric substrates which are bonded together, such as glass substrates, for example, and thereafter the peripheral edges thereof are sealed using a chosen seal material. The gap between two dielectric substrates is narrow and typically will measure less than 4 to 7 micrometers (.mu.m) for instance, which gap will be collectively referred hereinafter as a "cell gap". One prior known method of retaining this cell gap is to randomly distribute spherical spacers of substantially uniform diameter, sometimes called beads, between the substrates.
Although controllability of the cell gap may readily be enhanced by increasing the requisite number of beads that are distributed, the distribution amount has generally been set at 150 pieces per square millimeter in view of the fact that random distribution of such beads inherently lacks uniformity thereby making it very difficult to completely prevent some beads from locally crowding together at a location. This can result in an increase in the number of optical dot-like dislocations, and the random bead distribution also causes an adverse reaction, such as creation of an undesired disturbance in the alignment of the liquid crystals near or around such beads, which would result in a contrast reduction becoming greater locally.
While the beads may be made of an organic polymer or quartz, use of quartz beads can cause destruction of any one of the protective films, the electrodes, and the switching elements, such as TFTs, which are fabricated on a dielectric substrate at a press-machining step for establishment of the cell gap, or alternatively result in unwanted creation of air holes or "bubbles" with a change in temperature due to a difference in the thermal expansion coefficient between the beads and a liquid crystal material being used. For this reason organic polymer beads are employed in most cases.
In direct-view liquid crystal display devices, the beads which are distributed often attempt to move or "drift" upon application of a stress to the dielectric substrate. In this respect, it will be desirable for the liquid crystal layer to be kept at negative pressures relative to the atmospheric; however, presently available manufacturing technologies make it difficult to constantly maintain such a state in which the liquid crystal panel products are constantly held in a negative pressure condition.
On the other hand, small size liquid crystal display devices for use in projector equipment are burdened with a problem in that certain beads distributed between dielectric substrates of its liquid crystal panel, which reside in the panel's display area, can unintentionally be projected on a screen as a magnified shadow image, which in turn results in a decrease in the quality of the picture images being displayed. One prior known approach to avoiding such image quality reduction is to employ what is called a "beads-less" scheme which uses a limited number of beads or fibers only at the periphery of the liquid crystal panel's display area to thereby retain the intended cell gap at such periphery only. Unfortunately, this beads-less approach suffers from a difficulty in maintaining the cell gap in the display area at a predetermined value, which can result in a decrease in the production yield and in image quality.
Further, in recent years, high-speed image displayability has been demanded, which in turn calls for establishment of so-called "narrow gap" designs for further reduction of cell gaps with increased gap control accuracies of 0.1 .mu.m or below. As such narrow-gap designs are becoming more important, a need is felt to further increase the bead-spacer machining accuracy, which however is very difficult, especially in prior art reflective liquid crystal panels, wherein achievement of such high machining accuracy remains extremely difficult due to the fact that the cell gaps are nearly half the size of those in the devices of the transmission type.
One proposed approach to avoiding the cell-gap problem is to form, by photolithography techniques, columnar or pillar-shaped spacers (referred to hereinafter as "pole-like spacers") on a dielectric substrate at selected locations (certain portions that do not affect displayability, such as portions between adjacent pixels or alternatively those immediately underlying a black matrix) in the display area thereof, which spacers provide support between the two dielectric substrates stacked over each other to thereby render the cell gap uniform.
Use of such pole-like spacers eliminates local crowding and unwanted drift movement of distributed beads. Furthermore, as the fabrication accuracy of photolithography is significantly greater than the machining accuracy of beads by one order of magnitude or greater, the height of the pole spacers is simply determinable depending upon the thickness of the deposited photoresist film constituting these pole spacers, which in turn makes it possible to noticeably improve the cell gap accuracy.